The present invention relates generally to devices for transmitting ultrasonic waves and, more particularly, to such devices wherein long transmission paths are formed in or on the device by means of micromachined cavities.
Devices for transmitting ultrasonic waves have been used in a number of applications including electromechanical filters and ultrasonic delay lines. Recently, these devices have also been used in sensing viscosity of liquids and, with the proper coatings, the presence or absence of specific gases in gaseous mixtures. In general these devices utilize three different types of wave motions: (1) bulk shear wave motions; (2) high frequency surface wave motions; or (3) Lamb wave motions. The type of wave motion utilized is determined by the design details of the device transducers which are coupled to an ultrasonic wave transmission medium to transmit and receive the ultrasonic waves.
Devices for transmitting ultrasonic waves have been fabricated using a wide variety of materials as wave transmission media. Quartz, silicon and glass are examples of materials used as wave transmission media. The waves usually travel in the medium along a path which may be folded within the medium by reflective surfaces. Folding the path results in a relatively long transmission path in relation to the overall size of the transmission medium.
U.S. Pat. No. 4,684,906 discloses an ultrasonic delay line incorporating a solid wave transmission medium, such as glass. The device takes the form of a polygon with internally reflective surfaces formed on its outer surfaces. An input transducer is disposed on one surface with an output transducer on another of the reflective surfaces. Ultrasonic waves, travel from the input transducer through the medium to reflective surfaces which direct the waves through the medium ultimately to the output transducer to define an extended length path through the medium.
Folded path ultrasonic delay lines, as described in U.S. Pat. No. 3,041,556, also have been fabricated from disks of quartz material. Waves propagate through the disk in a folded path formed by specially oriented reflecting facets. The facets, ground around the edge of the disk, are positioned to reflect the waves back and forth across the major disk surface, thus defining a multiple-reflection, long transmission path.
Folded wave transmission paths have similarly been produced by utilizing reflecting surfaces positioned on the perimeter of a quartz plate. U.S. Pat. No. 3,581,247 discloses a delay line using the exterior surfaces of a polyhedron plate to reflect and guide the wave propagation.
While the known devices provide extended path lengths within a given device, all reflections take place at peripheral edges of the devices. This design is incompatible with the modern technique of batch fabrication developed for semiconductor processing. Accordingly, there is a need for an extended path length device which can be processed by batch fabrication techniques.